How can I make the font size small?
I have tried everything, but nothing helps...

Font size where? If you mean on specific forum elements, then go into your Admin CP -> Styles & Templates -> Style Manager -> (Choose the style you want to edit) -> Edit "All Style Options" and hit Go -> Change the font size to whatever you wish.

If you mean in your posts, then when you're posting click the Font Size drop-down menu and choose the font size you want (1 smallest - 7 largest). Hope this helps.

How can I make the font size small?
I have tried everything, but nothing helps...
There is a colormap reference made by jo on vbulletin.com/docs/html that could help determine which class to modify.